Cast aluminum alloy excellent in relaxation resistance property and method of heat-treating the same

ABSTRACT

A cast aluminum alloy excellent in the relaxation resistance property, comprising 9 to 17% by mass of Si, 3 to 6% by mass of Cu, 0.2 to 1.2% by mass of Mg, 0.2 to 1.5% by mass of Fe, 0.1 to 1% by mass of Mn, a balance consists of Al and unavoidable impurities, wherein a Ni content is not more than 0.5% by mass. The average hardness is adjusted to HV130 to HV160 by performing, after casting, solution heating by retaining the alloy at a treatment temperature of 450 to 510° C. for 0.5 hour or longer, performing water quenching and, thereafter, performing aging treatment by retaining the alloy at a treatment temperature of 170 to 230° C. for 1 to 24 hours.

CROSS-REFERENCES TO RELATED APPLICATIONS

The present application claims priority under 35 U.S.C. §119 to Japanese Application No. 2006-183919, filed Jul. 3, 2006, entitled “CAST ALUMINUM ALLOY EXCELLENT IN RELAXATION RESISTANCE PROPERTY AND METHOD OF HEAT-TREATING THE SAME”. The contents of this application are incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a cast aluminum alloy excellent in the relaxation resistance property and a method of heat-treating the same.

2. Background of the Invention

Since a cast aluminum alloy which is an aluminum alloy manufactured by casting has a relatively high strength while it is light, it is utilized in a variety of fields. Meanwhile, it is known that when an aluminum alloy is given a constant strain, and is allowed to stand, stress relaxation phenomenon is generated in which a stress generated by the strain is decreased with time. For this reason, when a cast aluminum alloy is used in an application in which relaxation phenomenon is easily generated, it is preferable that its material itself is excellent in the relaxation resistance property.

Among the previously known Al-Si-based cast aluminum alloys, some of them were studied regarding heat resistance, but there is little disclosure that the relaxation resistance property is studied and devised.

As a hypereutectic alloy of Al and Si having the enhanced heat resistance, for example, there is the technique described in Patent Document 1 regarding a cast aluminum alloy for a piston.

In addition, as a hypoeutectic alloy of Al and Si having the improved ductility and toughness, for example, there is the technique described in Non-Patent Document 1.

-   [Patent Document 1] JP 2004-76110 Unexamined Patent Publication     (Kokai) -   [Non-Patent Document 1] “Casting solidification”, published by the     Japan Institute of Metals on Jan. 20, 1992

The Patent Document 1 discloses a cast aluminum alloy containing 1.8 to 3% by mass of Ni. Like this example, in the conventional alloy, addition of Ni has been commonly performed in order to enhance heat resistance. However, since Ni is expensive, there is a strong need not to use Ni, if possible. In addition, action caused by inclusion of Ni on the relaxation resistance property in the present invention has previously been unknown. In addition, the aluminum alloy described in this Patent Document 1 has a hypereutectic structure of Al and Si.

The Non-Patent Document 1 shows addition of Sr, Na and Sb in order to finely distribute eutectic Si, in a hypoeutectic alloy of Al and Si. In addition, in the alloy described in this Document, eutectic Si is finely distributed aiming at improvement of ductility and toughness. Since ductility and toughness are reduced when a content of Cu is high, a Cu content is reduced.

In any event, previously, there has been little disclosure regarding a cast aluminum alloy which can improve the relaxation resistance property. Accordingly, an object of the present invention is to make clear a concept of a cast aluminum alloy more excellent in the relaxation resistance property than previously, and contribute to improvement in properties of a variety of parts.

SUMMARY OF THE INVENTION

A first aspect of the present invention is cast aluminum alloy excellent in the relaxation resistance property, comprising:

-   9 to 17% by mass of Si, -   3 to 6% by mass of Cu, -   0.2 to 1.2% by mass of Mg, -   0.2 to 1.5% by mass of Fe, -   0.1 to 1% by mass of Mn, and -   a balance consisting of Al and unavoidable impurities,

wherein a content of Ni is not more than 0.5% by mass, and

an average hardness is HV130 to HV160.

The cast aluminum alloy of the present invention is a cast aluminum alloy having the aforementioned specified composition, and having a hypoeutectic structure of Al and Si free from primary crystal Si. And, as described later, for example, by performing solution heating and water quenching after casting and, further, performing specified-time aging treatment at the specified temperature, its average hardness is adjusted to be HV130 to HV160. Thereby, a cast aluminum alloy more excellent in the relaxation resistance property than previously can be obtained.

That is, when the specified composition is selected, the extremely excellent relaxation resistance property has been obtained by, for example, performing the aging treatment in order to adjust the hardness into a specified range.

A second aspect of the present invention is a cast aluminum alloy excellent in the relaxation resistance property, comprising:

-   9 to 17% by mass of Si, -   3 to 6% by mass of Cu, -   0.3 to 1.2% by mass of Mg, -   0.2 to 1% by mass of Fe, -   0.1 to 1% by mass of Mn, -   0.15 to 0.3% by mass of Ti, and -   a balance consisting of Al and unavoidable impurities,

wherein a content of Ni is not more than 0.5% by mass, and

the alloy has an isotropic homogeneous structure in which a ratio of area of dendrite where 5 or more dendrite cells are aligned generally in one direction is not more than 20% in terms of an area ratio, and there is substantially no alignment of dendrites.

By having the specified composition mentioned above, the cast aluminum alloy of the second aspect of the present invention has a hypoeutectic structure free from primary crystal Si composed of an isotropic homogeneous structure in which there is substantially no alignment of dendrites as described above, and a eutectic region is present in a network manner. And, as described later, for example, by performing solution heating and water quenching after casting and, further, performing specified-time aging treatment at specified temperature, its average hardness is adjusted to be HV130 to HV160. Thereby, a cast aluminum alloy further excellent in the relaxation resistant property than previously can be obtained.

A third aspect of the present invention is a method of heat-treating a cast aluminum alloy excellent in the relaxation resistance property, containing:

-   9 to 17% by mass of Si, -   3 to 6% by mass of Cu, -   0.2 to 1.2% by mass of Mg, -   0.2 to 1.5% by mass of Fe, -   0.1 to 1% by mass of Mn, and -   a balance consists of Al and unavoidable impurities,

wherein a content of Ni is not more than 0.5% by mass, and

the method comprising:

performing, after casting, solution heating by retaining the alloy at a treatment temperature of 450 to 510° C. for 0.5 hour or longer,

performing water quenching and, thereafter,

performing aging treatment by retaining the alloy at a treatment temperature of 170 to 230° C. for 1 to 24 hours, thereby, adjusting an average hardness at HV130 to HV160.

The heat-treatment method of the third aspect of the present invention is a heat-treatment method for obtaining the aforementioned cast aluminum alloy of the first aspect of the present invention and, by implementing this, the aforementioned cast aluminum alloy excellent in the relaxation resistance property can be obtained.

A fourth aspect of the present invention is a method of heat-treating a cast aluminum alloy excellent in the relaxation resistance property, containing:

-   9 to 17% by mass of Si, -   3 to 6% by mass of Cu, -   0.3 to 1.2% by mass of Mg, -   0.2 to 1% by mass of Fe, -   0.1 to 1% by mass of Mn, -   0.15 to 0.3% by mass of Ti, and -   a balance consists of Al and unavoidable impurities,

wherein a content of Ni is not more than 0.5% by mass, and

the method comprising:

performing, after casting, solution heating by retaining the alloy at a treatment temperature of 450 to 510° C. for 0.5 hour or longer,

performing water quenching and, thereafter,

performing aging treatment by retaining the alloy at a treatment temperature of 170 to 230° C. for 1 to 24 hours, thereby, adjusting an average hardness at HV130 to HV160.

The heat-treatment method of the fourth present invention is a heat-treatment method for obtaining the aforementioned cast aluminum alloy of the second aspect of the present invention and, by implementing this, the aforementioned cast aluminum alloy further excellent in the relaxation resistance property can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is illustration showing a construction of an instrument for performing a relaxation resistance property test.

FIG. 2 is illustration showing the state where a bending stress in a relaxation resistance property test is applied.

FIG. 3 is illustration showing the state where a bending stress in a relaxation resistance property test is released.

FIG. 4 is illustration showing a construction of an apparatus for performing a relaxation resistance property test.

FIG. 5 is a microphotograph at magnification×100 showing a metal structure of Example 6.

FIG. 6 is a microphotograph at magnification×400 showing a metal structure of Example 6.

FIG. 7 is a microphotograph at magnification×100 showing a metal structure of Example 8.

FIG. 8 is a microphotograph at magnification×400 showing a metal structure of Example 8.

FIG. 9 is a microphotograph at magnification×100 showing a metal structure of Comparative Example 2.

FIG. 10 is a microphotograph at magnification×400 showing a metal structure of Comparative Example 2.

FIG. 11 is a microphotograph at magnification×100 showing a metal structure of Comparative Example 1.

FIG. 12 is a microphotograph at magnification×400 showing a metal structure of Comparative Example 1.

FIG. 13 is a microphotograph at magnification×100 showing a metal structure of Comparative Example 9.

FIG. 14 is a microphotograph at magnification×400 showing a metal structure of Comparative Example 9.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

First, reasons why each alloy element amount in the first and third aspect of the present inventions is limited will be shown.

Si: 9 to 17% By Mass

Si is an essential element for maintaining castability, suppressing thermal expansion, and maintaining the relaxation resistance property. When a Si addition amount is less than 9% by mass, a eutectic Si amount is small, and a network skeleton for suppressing relaxation is not sufficiently formed. On the other hand, when a Si content exceeds 17% by mass, since a liquidus temperature is remarkably elevated, and a casting temperature must be elevated, a problem such as gas absorption, oxidation, and consumption of a mold is occurred, and it is not preferable.

It is preferable that the alloy has a hypoeutectic structure in which a Si content is 9 to 12% by mass and primary crystal Si is not present. A further optimal range of a Si content is 10 to 11% by mass. When a Si content exceeds 12% by mass, coarse primary crystal Si is produced, and in fatigue under a high tensile average stress, the coarse primary crystal Si is destructed, and a fatigue strength is reduced in some cases.

Cu: 3 to 6% By Mass

Cu is an element effective in producing a precipitate containing Cu to improve a strength of an alloy. Particularly, there is great contribution to improvement in a strength at a high temperature. When the content is less than 3%, the strength improving effect is small and, when the content exceeds 6%, solidification segregation is great, and a homogeneous material can not be obtained. In addition, there is a fear that ductility is remarkably reduced, and a fatigue strength under a tensile average stress is reduced. Preferably, a Cu content is 4 to 5% by mass.

Mg: 0.2 to 1.2% By Mass

Mg produces a Mg₂Si-based precipitate, and improves a strength by the precipitation strengthening. In addition, Mg produces crystallized Mg₂Si, and improves a strength by dispersion-strengthening due to a crystallized substance. When a Mg content exceeds 1.2% by mass, since a crystallization amount of Mg₂Si is too large, toughness is reduced, and a disadvantage arises such that a fatigue strength is reduced. When the content is less than 0.2% by mass, a precipitation amount is small, and a fatigue strength is not sufficient.

Preferably, a Mg content is 0.6 to 1% By Mass. Fe: 0.2 to 1.5% By Mass

Fe exerts the effect of forming a crystallized substance having the high heat resistance, and suppressing relaxation by uniformly dispersing this crystallized substance with crystallized Si or dispersing this in a network manner. When a Fe content is less than 0.2%, the effect is small and, when a Fe content exceeds 1.5%, there is a fear that a coarse crystallized substance is formed, and the crystallized substance becomes a fracture origin, resulting in reduction in a fatigue strength under a tensile average stress. In addition, Fe contributes to improvement in resistance of seizure with a mold. Preferably, a Fe content is 0.3 to 1% by mass.

Mn: 0.1 to 1% By Mass

Mn is not an essential element to be added, but since Mn contributes to form a crystallized substance having the high heat resistance like Fe, and to improve the heat resistance of the base aluminum phase to suppress relaxation and, at the same time, contributes to improvement in resistance of seizure with a mold, it is preferable to add Mn. When a Mn content is less than 0.2% by mass, that effect is small and, when a Mn content exceeds 1% by mass, there is a fear that a coarse crystallized substance is formed, and the crystallized substance becomes a fracture origin, resulting in reduction in a fatigue strength under a tensile average stress. Preferably, a Mn content is 0.2 to 0.7% by mass.

Ni: Not More Than 0.5% By Mass

Since Ni forms a coarse crystallized substance, this renders a structure heterogeneous, relaxation is easily generated. Therefore, Ni is restricted to a range of not more than 0.5% by mass. Particularly, since when a Cu content is high, a coarse crystal containing Cu and Ni is easily formed, and it is not preferable to add Ni. In addition, addition of Ni remarkably increases a density of an alloy. And, when a Ni content exceeds 0.5% by mass, there is a problem that a coarse crystallized part is formed, relaxation is easily generated and, at the same time, a density is increased, and a product becomes heavy.

Then, in the first aspect of the present invention, it is preferable that the average hardness is adjusted by performing, after casting, solution heating by retaining the alloy at a treatment temperature of 450 to 510° C. for 0.5 hour or longer, performing water quenching and, thereafter, performing aging treatment by retaining the alloy at a treatment temperature of 170 to 230° C. for 1 to 24 hours. In addition, this is essential in the third aspect of the present invention.

That is, an average hardness of a cast aluminum alloy having the chemical composition described in the first and the third aspect of the inventions is not less than HV162 when conventional T6 treatment aiming at a strength is performed, and is not more than HV120 unless heat treatment is performed. By performing the specified solution treatment and aging treatment to positively adjust an average hardness at HV130 to HV160, the relaxation resistance property can be improved.

An average hardness is obtained by measuring a Vickers hardness of 5 or more portions free from a casting defect under the condition of a load of 10 kgf and a retention time of 30 seconds at a steady state part of a cross-section of a cast aluminum alloy, and calculating an average value thereof. In addition, when a structure is fine, and a scatter due to indentation positions is small, approximately the same value is obtained at a load of 5 kgf.

When the average hardness is less than HV130, there is a problem that a strength is low and, when a rapid load is applied at a part, the part is easily deformed. On the other hand, when the average hardness exceeds HV160, there is a problem that relaxation is easily occurred. The average hardness is preferably HV140 to HV160, more preferably HV150 to HV160.

A concept that, when a hardness is slightly lowered than a maximum value at peak aging, relaxation is hardly occurred, has not previously been present, and was first found out in the present invention. This mechanism has still some unknown points, but is presumed as follows.

Relaxation is a phenomenon in which a stress is reduced accompanied with retention at a high temperature. It is thought that, in the alloy of the present invention containing Cu and Mg, a precipitate produced by heat treatment is present in a base aluminum phase, this precipitate possesses effects of suppressing sliding deformation in a base aluminum phase, and effects of hardly giving rise to relaxation. This was preciously thought as follows. Since as this precipitate is finer and distributed more densely, the effect of suppressing sliding deformation is greater, an alloy having hardness enhanced at peak aging has a finer and more densely distributed precipitate. Therefore, relaxation is hardly generated.

However, when retained at a high temperature, a coarser precipitate is produced, and it is thought that, under the stress loading state, this coarser precipitate is produced so as to degrease a stress. This is termed stress aging. On the other hand, in the case of an alloy in which a hardness is reduced from the hardness at peak aging with excessive aging treatment, a change in a precipitate is small even when heated at the same temperature. That is, it is thought that, since in an alloy having a slightly reduced hardness, a change in a structure of a precipitate is smaller, and decrease in a stress accompanying with structure change is less likely generated, relaxation is hardly generated.

The condition of the solution treatment is such that a treatment temperature for solution heating is in a range of 450 to 510° C., and a retention time is in a range of 0.5 hour or longer. When a treatment temperature for solution heating is lower than 450° C., there is a problem that it takes an extremely long time to form a supersaturated solid solution of Cu or Mg and, in a short treatment time in the aforementioned range, a proper strength is not obtained. On the other hand, when the treatment temperature exceeds 510° C., there is a problem that burning to melt a part of the alloy is generated, and a pore defect is generated. In addition, when a retention time is shorter than 0.5 hour, there is a problem that sufficient solution treatment can not be performed. For this reason, it is preferable that a retention time is 1 hour or longer. On the other hand, when the retention time exceeds 10 hours, since a change does not occur even when retained longer, productivity is reduced.

Cooling water used in the water quenching may be normal water, or water to which a certain additive has been added, and various cooling waters can be applied as far as they are the known cooling water for water quenching.

The condition of the aging treatment is such that a treatment temperature is in a range of 170 to 230° C., and a retention time is in a range of 1 to 24 hours. When a treatment temperature for aging treatment is lower than 170° C., there is a problem that a hardness becomes too high, or a hardness is further increased during use. On the other hand, when a treatment temperature exceeds 230° C., there is a problem that a hardness is reduced more than necessary, and a strength is remarkably reduced. In addition, when the retention time is shorter than 1 hour, there is a problem that sufficient age hardening is not obtained and, on the other hand, when the retention time exceeds 24 hours, there is a problem that a hardness is reduced too much, or a hardness is saturated, and productivity is reduced. A preferable aging temperature is 190 to 210° C., and an optimal aging temperature is 190 to 200° C.

Then, the second aspect of the present invention has the greatest characteristic that a ratio of dendrites in which 5 or more of dendrite cells are aligned generally in one direction is not more than 20% in terms of an area ratio, there is substantially no alignment of dendrites, and an isotropic homogeneous structure in which a eutectic region is present in a network manner, or crystallized substances are uniformly distributed, thereby, the relaxation resistance property can be further improved.

That is, it is thought that since by controlling the alloy compositions and a structure form, a firm dispersion-strengthened structure suppressing high temperature deformation is formed and, at the same time, high temperature deformation is suppressed by a heat resistance improving composition present in a base Al phase, the relaxation resistance property of the alloy is improved. In addition, it is thought that since a crystallized substance is isotropically dispersed and strengthened, a stress distribution becomes uniform, and a fatigue strength is improved.

Further, since expensive Ni is contained little, the material cost can be reduced. In addition, inclusion of little Ni suppresses generation of a coarse crystallized substance, and the crystallized substance can contribute to improvement in a fatigue strength and the relaxation resistance property efficiently, by isotropically dispersing fine crystallized substances having uniform sizes. Particularly, that effect becomes remarkable under the high temperature environment of not higher than 200° C.

The isotropic homogeneous structure in which a eutectic region is present in a network manner, or crystallized substances are uniformly dispersed can be realized by substantially having no alignment of dendrites. In the present invention, the state where there is substantially no alignment of dendrites is defined as the case where a ratio of an area of a dendrite structure in which 5 or more dendrite cells are aligned generally in one direction (hereinafter, conveniently referred to as aligned dendrite structure) is not more than 20% of an entire area of a structure.

More preferably, the area ratio of the aligned dendrite structure is not more than 10% and, most preferably, the area ratio of the aligned dendrite structure is not more than 5%.

In order to obtain such isotropic homogenous structure, it is necessary to make the aforementioned specified composition be essentially requirement. Reasons why each alloy element amount in the cast aluminum alloy of this second aspect of the present invention (fourth aspect of the present invention) is limited will be shown.

Si: 9 to 17% By Mass

Si is an essential element for forming a network skeleton of eutectic Si. When a Si content is less than 9.5%, since a eutectic Si amount is small, and a network becomes incomplete, there is a fear that the relaxation property is reduced. In addition, when a Si content exceeds 17% by mass, since a liquidus temperature is remarkably elevated, and a casting temperature must be elevated, a problem such as gas absorption, oxidation, and consumption of a mold is occurred, and it is not preferable. It is preferable that a Si content is preferably 9 to 12% by mass. A range of a further optimal a Si content is 10 to 11% by mass. When a Si content exceeds 12% by mass, coarse primary crystal Si is produced and, in fatigue under a high tensile average stress, coarse primary crystal Si is fractured, and a fatigue strength is reduced in some cases.

Cu: 3 to 6% By Mass

Cu is an element effective in producing a precipitate containing Cu to improve a strength of an alloy. Particularly, there is great contribution to improvement in a strength at a high temperature. When the content is less than 3%, the strength improving effect is small and, when the content exceeds 6%, solidification segregation is great, and a homogeneous material can not be obtained. In addition, there is a fear that ductility is remarkably reduced, and a fatigue strength under a tensile average stress is reduced. Preferably, a Cu content is 4 to 5% by mass.

Mg: 0.3 to 1.2% By Mass

Mg produces a Mg₂Si-based precipitate, and improves a strength by the precipitation strengthening. In addition, Mg produces crystallized Mg₂Si, and improves a strength by dispersion-strengthening due to the crystallized substance. When a Mg content exceeds 1.2% by mass, since a crystallization amount of Mg₂Si is too large, toughness is reduced, and a disadvantage arises such that a fatigue strength is reduced. When the content is less than 0.3% by mass, a precipitation amount is small, and fatigue strength is not sufficient. A Mg content is preferably 0.4 to 1% by mass, further preferably 0.6 to 1% by mass.

Fe: 0.1 to 1% By Mass

Fe forms a crystallized substance having a high heat resistance, strengthens a network skeleton consisting of crystallized substances, and contributes to improvement in the relaxation resistance property. When a Fe content is less than 0.1%, the effect is small and, when a Fe content exceeds 1%, there is a fear that a coarse crystallized substance is formed, and the crystallized substance becomes a fracture origin, resulting in reduction in a fatigue strength under a tensile average stress. In addition, Fe contributes to improvement in resistance of seizure with a mold. Preferably, a Fe content is 0.3 to 1% by mass.

Mn: 0.1 to 1% By Mass

Mn, by addition, forms a crystallized substance having a high heat resistance, strengthens a network skeleton consisting of crystallized substances, contributes to improvement in the relaxation resistance property and, at the same time, contributes to improvement in resistance of seizure with a mold. When a Mn content is less than 0.1% by mass, that effect is small and, when a Mn content exceeds 1%, there is a fear that a coarse crystallized substance is formed, and the crystallized substance becomes a fracture origin, resulting in reduction in a fatigue strength under a tensile average stress. Preferably, a Mn content is 0.2 to 1% by mass.

Ti: 0.15 to 0.3% By Mass

Ti has the effect of finely distributing crystal particles of an α-Al phase, and suppressing alignment of dendrite cells to homogenize solidified structure, and the effect of improving heat resistance of a base aluminum phase, and improves the relaxation resistance property of the same phase.

When a Ti content is less than 0.15% by mass, a solidified structure is homogenized and, in the case of a hypoeutectic structure, a network skeleton structure consisting of crystallized substances is not formed isotropically. In addition, in the case of a hypereutectic structure, an isotropic uniformly dispersed structure of crystallized substances is not formed. Further, a Ti amount in a base aluminum phase is low, and the relaxation resistance property of the same phase is not sufficiently obtained.

When a Ti content exceeds 0.3% by mass, there is a possibility that a coarse Ti compound is produced, toughness is reduced and, at the same time, the compound becomes an origin of fatigue fracture, resulting in reduction in a fatigue strength under a tensile average stress.

When addition of Ti is performed by an addition of Al—Ti—B alloy or Al—Ti—C alloy, B and C are permitted to be contained. A range of a preferable Ti content is 0.15 to 0.25% by mass. When a Ti content is not less than 0.15% by mass, homogeneity of a structure and isotropy of a network skeleton structure are increased by sufficiently finely distributed crystal particle, the relaxation resistance property becomes higher and, at the same time, a scatter of a fatigue strength becomes smaller, resulting in improvement in a lower limit value of a fatigue strength. A further optimal Ti content is 0.2 to 0.25% by mass. In this range, the relaxation resistance property becomes highest.

Ni: Not More Than 0.5% By Mass

Since Ni forms a coarse crystallized substance, and to form heterogeneous structure, relaxation is easily generated, therefore, an amount of Ni is restricted to a range of not more than 0.5% by mass. In particular, when a Cu content is high, since a coarse crystal containing Cu and Ni is easily formed, it is not preferable to add Ni. And, addition of Ni remarkably increases a density of an alloy. And, when a Ni content exceeds 0.5% by mass, there is a problem that a coarse crystallized part is formed, relaxation is easily generated and, at the same time, a density is increased, resulting in a heavy product.

It is preferable that the cast aluminum alloy of the second and fourth aspect of the present inventions further contains 0.05 to 0.15% by mass of Zr, and 0.02 to 0.15% by mass of V.

Zr: 0.05 to 0.15% By Mass

Zr, like Ti, has the effect of finely distributing crystal particles of an α-Al phase, and suppressing alignment of dendrite cells to homogenize a solidified structure, and the effect of enhancing heat resistance of a base aluminum phase, and improving the relaxation resistance property. In order to obtain sufficient homogeneity and heat resistance of a solidified structure, it is preferable to contain not less than 0.05% by mass of Zr. When a Zr content is less than 0.05% by mass, there is a possibility that sufficient fine distribution of crystal particle for homogenizing a solidified structure can not be attained. In addition, a content in a base aluminum phase is low, and there is a possibility that sufficient heat resistance is not obtained. When a Zr content exceeds 0.3% by mass, there is a possibility that a coarse Zr compound is produced, and becomes a fatigue origin. Further, when Zr is used with Ti, the effect is further enhanced.

V: 0.02 to 0.15% By Mass

V is present mainly in a base aluminum phase, and has an effect of improving the relaxation resistance property due to improvement in heat resistance. When V is contained at not less than 0.02% by mass, it is preferable the effect is clearly expressed. In order that V is contained at an amount exceeding 0.15% by mass, a melting temperature becomes higher, and a problem of gas absorption or the like arises, so it is not desirable. In addition, there is a possibility that a coarse V compound is produced, and becomes an origin of a fatigue fracture. A V content is preferably 0.02 to 0.12% by mass. Further, when V is used with Ti, heat resistance of a base aluminum phase becomes highest, and it becomes optimal.

In addition, when Ti, Zr and V are contained together, the most excellent relaxation resistance property is obtained by the aforementioned synergistic effect.

In the first and second aspect of the present inventions, it is preferable to adjust a composition so that a density of the cast aluminum alloy becomes not higher than 2.8 g/cm³. Thereby, the lightening effect due to adoption of an aluminum alloy can be further enhanced.

It is preferable that the cast aluminum alloy in the second and fourth aspect of the present inventions has a hypoeutectic structure in which a Si content is 9 to 12% by mass, a P content is not more than 0.001% by mass, and primary crystal Si is not present. That is, when the alloy is of a hypoeutectic structure, it is preferable to limit a P content to 0.001% by mass or less.

When a large amount of P is contained, there is a fear that a eutectic point of an alloy is shifted, a coarse primary crystal Si is produced in a composition range of the alloy of the present invention, and it becomes an origin of fatigue fracture, resulting in reduction in a fatigue strength under a tensile average stress. For this reason, it is preferable that a P content is not more than 0.001% by mass, and ideally 0.

On the other hand, in the case of a hypereutectic structure, it is preferable that a P content is 0.005 to 0.015% by mass. By inclusion of P, primary crystal Si is finely distributed, and a fatigue strength under a tensile average stress is improved. When a P content is less than 0.005% by mass, fine distribution of primary crystal Si becomes insufficient. On the other hand, even when P is contained at an amount exceeding 0.015% by mass, the effect is saturated, and harmful effect such as poor fluidity is likely occurred, and it is not preferable.

It is preferable that the cast aluminum alloy in the second and fourth aspect of the present inventions further contains at least one member selected from the group consisting of:

-   0.0005 to 0.01% by mass of Ca, -   0.0005 to 0.003% by mass of Na, -   0.003 to 0.03% by mass of Sr, and -   0.05 to 0.2% by mass of Sb.

Ca: 0.0005 to 0.01% By Mass

Since Ca finely distributes eutectic Si, forms a wide network skeleton consisting of fine Si, and affords the effect of suppressing relaxation, it is preferable to add Ca. When a Ca content is less than 0.0005% by mass, there is a problem that fine distribution of eutectic Si is insufficient. On the other hand, when a Ca content exceeds 0.01% by mass, there are problems that a molten metal is easily oxidized, an oxide is mixed into a casting, or gas absorption is increased, resulting in increase in a pore defect.

Na: 0.0005 to 0.003% By Mass

Since Na finely distributes eutectic Si and affords the same effect as that of Ca, it is preferable to add Na. When a Na content is less than 0.0005% by mass, there is a problem that fine distribution of eutectic Si is insufficient. On the other hand, when a Na content exceeds 0.003% by mass, there is a problem that gas absorption is increased, resulting in increase in a pore defect.

Sr: 0.003 to 0.03% By Mass

Since Sr finely distributes eutectic Si and affords the same effect as that of Ca, it is preferable to add Sr. When a Sr content is less than 0.003% by mass, there is a problem that fine distribution of eutectic Si is insufficient. On the other hand, when a Sr content exceeds 0.03% by mass, there is a problem that gas absorption is increased, resulting in increase in a pore defect.

Sb: 0.05 to 0.2% By Mass

Since Sb finely distributes eutectic Si and affords the same effect as that of Ca, it is preferable to add Sb. When a Sb content is less than 0.05% by mass, there is a problem that fine distribution of eutectic Si is insufficient. On the other hand, when a Sb content exceeds 0.2% by mass, there is a problem that gas absorption is increased, resulting in increase in a pore defect.

In addition, since Na has a problem that it reacts with a mold coating material of a furnace wall to likely damage the furnace wall, Sr has a problem that gas absorption is easily generated, and Sb has a problem that the effect of finely distributing eutectic Si is relatively small. Therefore, it is most preferable that Ca is contained.

Also in the cast aluminum alloy in the second aspect of the present invention, it is preferable that an average hardness is HV130 to HV160 as described above.

In addition, in the cast aluminum alloy, it is preferable that the average hardness is adjusted by performing, after casting, solution heating by retaining the alloy at a treatment temperature of 450 to 510° C. for 0.5 hour or longer, performing water quenching and, thereafter, performing aging treatment by retaining the alloy at a treatment temperature of 170 to 230° C. for 1 to 24 hours. In the fourth aspect of the present invention, this is essential.

Thereby, in conjunction with realization of the aforementioned isotropic homogeneous structure, the relaxation resistance property can be further improved.

In the second aspect of the present invention, it is preferable that an average longitudinal diameter of crystallized Si in the cast aluminum alloy is not more than 5 μm. The crystallized Si referred in this context is mainly eutectic Si and a smaller particle diameter is preferable. When an average longitudinal diameter of the crystallized Si exceeds 5 μm, there is a possibility that sliding deformation is generated at an interface between a crystallized substances and a base aluminum phase, and it is difficult to suppress relaxation. An average longitudinal diameter of the crystallized Si is preferably not more than 4 μm, more preferably not more than 3 μm.

It is preferable that the cast aluminum alloy in the first to fourth aspect of the present inventions further contains at least one member selected from the group consisting of:

-   0.01 to 0.1% by mass of Sn, -   0.02 to 0.15% by mass of Pb, and -   0.1 to 1% by mass of Zn.

Sn: 0.01 to 0.1% By Mass

Since a recycled ingot of Sn can be used as a raw material by permitting inclusion thereof, the recycling property is improved. Therefore, it is preferable that Sn is permitted to be contained. When a Sn content is restricted to less than 0.01% by mass, there is a problem that a recycled alloy can not be used, and a scope of a raw material is limited. On the other hand, when a Sn content exceeds 0.1% by mass, there is a problem that heat resistance is reduced and relaxation is likely generated.

Pb: 0.02 to 0.15% By Mass

Since Pb affords the effect of improving the recycling property like Sn, it is preferable that Pb is permitted to be contained. When a Pb content is less than 0.02% by mass, there is a problem that a scope of a raw material to be used is restricted. On the other hand, when a Pb content exceeds 0.15% by mass, there is a problem that heat resistance is reduced, and relaxation is likely generated.

Zn: 0.1 to 1% By Mass

Since Zn affords the effect of improving the recycling property like Sn and Pb, it is preferable that Zn is permitted to be contained. When a Zn content is less than 0.1% by mass, there is a problem that a scope of a raw material to be used is restricted. On the other hand, when a Zn content exceeds 1% by mass, there is a problem that heat resistance is reduced, and relaxation is likely generated.

It is preferable that a Si amount in a base Al phase in the cast aluminum alloy is not less than 0.95% by mass. Thereby, sliding deformation of the base aluminum phase is suppressed, and the effect of less likely generating relaxation can be obtained.

In this context, a concept of the invention in the present application will be described briefly.

Since relaxation is a phenomenon intimately connected with creep, it has been thought that a material excellent in the creep property is also excellent in the relaxation property. In other words, generally, it is presumed that a material having a high proof stress and strength is also excellent in the relaxation property.

On the contrary, in the cast aluminum alloy of the present invention, for the first time, it was found out that a material having a greatest strength or proof stress is not most excellent in the relaxation property. In other words, it was found out for the first time that a material in which a hardness is slightly lowered by performing excessive aging treatment that is slightly reducing a strength rather than the normal T6 heat treatment generating a peak strength is less likely to generate relaxation than the previous material having a peak hardness.

Further, as the finding previously obtained in study on a heat resistant magnesium alloy that is likely generating relaxation and having this as a practical great problem, it is known that deformation in grains is restricted with a network skeleton in which heat resistant particles are continuously connected, and relaxation is suppressed. However, in the present invention, it was first found out that the skeleton may not be continuous, and relaxation can be more effectively suppressed by forming a wide network skeleton region with an aggregation of fine particles.

In addition, the network is isotropic and, further, by rendering a base aluminum phase surrounded by the network difficult to slip by a heat resistant element, an entire structure is optimally designed so as to less likely generate relaxation by multiscale structure control in which microstructure control and macrostructure control are fused, thereby, the high relaxation resistance property which has not previously been seen could be first realized.

In addition, it was also first found out that, by such the optimal design, even when a composition such as Sn, Pb and Zn which are a harmful element as a low melting point metal, easily generating relaxation is contained, the harm is eliminated and the effect of maintaining the excellent relaxation resistance property can be obtained. This effect of rendering harmless dramatically enhances the recycling property of an alloy and, since an alloy which has an extremely low environmental impact, and an extremely high practical value can be provided, an industrial utilization value is extremely high.

EXAMPLES

Cast aluminum alloys related to Examples of the present invention will be explained.

In present Examples, various cast aluminum alloys (Examples 1 to 9, Comparative Examples 1 to 10) shown in Table 1 to Table 3 were manufactured, and properties thereof were assessed.

A group shown in Table 1 is an example concerning the first aspect of the present invention, a group shown in Table 2 is an example concerning the second aspect of the present invention, and a group shown in Table 3 is an example concerning the second aspect of the present invention in which essential compositions are increased on the assumption that a recycled metal is utilized.

Aluminum alloys having chemical compositions shown in Table 1 to Table 3 were prepared by melting. In any case, a molten metal in which compositions were adjusted was prepared, a flux was added to the molten metal at 740 to 760° C. to perform deoxidation, this was subjected to degassing treatment with hexachloroethane and Ar gas bubbling, and resulting molten metal was cast into a boat-shape mold pre-heated at 200° C. for taking a JIS No. 4 type test piece or a Ransley copper mold. A casting temperature was 730° C. The resulting casting material was solution-heated by retaining at 500° C. for 3 hours, subjected to water quenching and, then subjected to aging treatment under the condition shown in each Table.

(Table 1)

TABLE 1 aging condition chemical compositions (mass %) (temperature · sample Si Cu Mg Fe Mn Ni Ti Zr V P Ca Na Pb Sn Zn time) Comparative 10.2 2.01 0.18 0.79 0.25 0.04 0.06 0.02 0.62 200° C. 5 h Example 1 Comparative 10.5 4.59 0.79 0.55 0.26 0.00 0.01 0.002 0.01 180° C. 8 h Example 2 Comparative 12.8 3.11 0.71 0.44 0.41 2.08 0.22 0.1 0.05 0.011 0.01 210° C. 3 h Example 3 Comparative 11.0 3.20 0.70 0.40 0.40 2.00 0.20 0.1 0.05 0.01 210° C. 3 h Example 4 Comparative 11.0 3.20 0.70 0.40 0.40 2.00 0.20 0.1 0.05 0.01 180° C. 5 h Example 5 Comparative 11.0 3.20 1.50 0.40 0.40 2.00 0.20 0.1 0.05 0.01 180° C. 5 h Example 6 Comparative 10.2 2.01 0.18 0.79 0.25 0.04 0.06 0.02 0.62 180° C. 5 h Example 7 Comparative 11.0 6.50 0.70 0.40 0.40 0.00 0.01 0.01 180° C. 5 h Example 8 Example 1 10.5 4.59 0.79 0.55 0.26 0.00 0.01 0.01 200° C. 5 h Example 2 11.0 3.80 0.80 0.40 0.40 0.00 0.01 0.01 200° C. 5 h Example 3 11.0 5.30 0.70 0.40 0.40 0.00 0.01 0.01 200° C. 5 h

(Table 2)

TABLE 2 aging chemical compositions (mass %) condition sample Si Cu Mg Fe Mn Ni Ti Zr V P Ca Na Pb Sn Zn (temperature · time) Example 4 10.5 4.60 0.80 0.55 0.26 <0.01 0.22 0.1 0.05 0.001 200° C. 5 h Example 5 10.5 4.60 0.80 0.55 0.26 <0.01 0.22 0.1 0.05 0.001 220° C. 6 h Example 6 10.5 4.60 0.80 0.55 0.26 <0.01 0.22 0.1 0.05 0.003 200° C. 5 h Example 7 10.5 4.60 0.80 0.55 0.26 <0.01 0.22 0.1 0.05 0.001 200° C. 5 h Example 8 10.5 4.60 0.80 0.40 0.40 <0.01 0.20 0.1 0.05 0.003 200° C. 5 h Comparative 12.5 4.60 0.80 0.55 0.40 3.00 0.22 0.1 0.05 0.010 200° C. 5 h Example 9

(Table 3)

TABLE 3 chemical compositions (mass %) aging condition sample Si Cu Mg Fe Mn Ni Ti Zr V P Ca Na Pb Sn Zn (temperature · time) Example 9 10.5 4.50 0.80 0.73 0.23 0.04 0.20 0.1 0.05 0.003 0.05 0.02 0.57 200° C. 5 h Comparative 10.5 4.60 0.80 0.72 0.23 0.04 0.20 0.1 0.05 0.003 0.30 0.20 0.56 200° C. 5 h Example 10

From the thus heat-treated casting material, a sample for microstructural observation and a test piece for assessing the relaxation resistance property were taken by mechanical processing.

Details of each test piece and a test method are as follows.

<Relaxation Resistance Property Assessment Test>

The dimension of test piece for assessing the relaxation resistance property was width 10 mm×thickness 1.3 mm×length 55 mm, and was taken so that a width direction accords to an upper and lower direction from a bottom part of a boat-shape material.

Assessment of the relaxation resistance property was performed using the following relaxation test instrument 1 as shown in FIG. 1 to FIG. 4.

The relaxation test instrument 1, as shown in FIG. 1, has a support member 3 held between one pair of test pieces 11, 12, a restriction member 4 for restricting both end parts 111, 112, 121, 122 of test pieces 11, 12, and high temperature strain gauges 21, 22 as a displacement detection means for detecting a strain displacement generated in test pieces 11, 12.

As shown in the same figure, the support member 3 is a round bar made of SUS304 excellent in the heat resistance, and its outer circumferential side 300 has an arc shape. The support member 3 has a dimension of a diameter of 6 mm and a length of 25 mm.

The restriction member 4 is constructed of two bolts 41, 42, and nuts 413, 423 threading with bolts 41, 42. In addition, washers 415, 416, 425, 426 for securing and stabilizing, and relaxation-preventing nuts 413, 423 were also used. The restriction member 4 is such that all members are made of SUS 304 excellent in the heat resistance as in the support member 3.

And, in the present Example, as shown in FIG. 1 to FIG. 3, after test pieces 11, 12 are faced in the state where a displacement measuring part is faced outwardly and, at the same time, the support member 3 is held with central parts 110, 120 between both of test pieces 11, 12, test pieces 11, 12 are restricted with the restriction member 4 to apply a predetermined bending stress and, at the same time, a strain displacement generated in test pieces 11, 12 by the bending stress is retained constant.

The relaxation test apparatus 5, as shown in FIG. 4, is connected to the relaxation test instrument 1, a heating tank 51 for heating test pieces 11, 12, and high temperature strain gauges 21, 22, and has a multichannel-type static strain meter 52 as a strain measuring equipment for measuring a strain displacement generated in test pieces 11, 12. As the heating tank 51, a warm air circuration-type constant temperature chamber (set temperature 50 to 300° C., temperature distribution within ±5° C.) was used.

As shown in same figure, gauge leads 211, 212 (221, 222) are connected to the high temperature strain gauge 21 (22), lead wires 521 to 523 (524 to 526) having a small electric resistance are connected to the static strain meter 52, and both are connected by soldering at a connection part 210.

A relaxation test, as shown in FIG. 2, is performed by a heating step of heating a test piece for a predetermined time while a strain displacement generated in test pieces 11, 12 by a bending stress is retained constant, and a step of, thereafter as shown in FIG. 3, after cooling, releasing a bending stress applied to test pieces 11, 12, and detecting a strain displacement generated in test pieces 11, 12 thereupon with high temperature strain gauges 21, 22 to obtain a residual stress. In the present Example, a residual stress σr was measured with a test temperature (heating temperature) of 180° C., and an initial loading stress of 200 MPa, for the test piece retained at 180° C. for 300 hours.

<Microstructure Observation>

A microstructure was observed at a location at a height of 14 mm from a bottom of the boat-shape material, which is the same location as a parallel part of a fatigue test piece. An area ratio Adp (%) of a dendrite structure (aligned dendrite structure) in which 5 or more dendrite cells are aligned generally in one direction was obtained specifically as follows: in a microphotograph of a field of about 1.4×1 mm square observed with a optical microscope at magnification×100, a part of a dendrite structure in which 5 or more dendrite cells are aligned in one direction was entirely painted, and an area ratio of the painted part was obtained by image processing software.

In addition, an average longitudinal diameter DsL (μm) of crystallized Si was obtained as follows. Lengths of a longest straight lines connecting two points on an outer circumference of particles, which pass through gravity center of the particles were obtained for all observed particles, and an average value thereof was calculated.

<Hardness Test>

A test piece for a hardness test was cut from a position at a height of about 14 mm from a bottom of a boat-shape casting, and a surface thereof was subjected to mirror polishing. And, an indentation was made on a part having no casting defect under the condition of load of 10 kgf and a loading time of 30 seconds, and an average of 5 or more normal measured values except for an abnormal value undergoing influence of a casting defect was taken, and a Vickers hardness HV as an average hardness was obtained by such a procedure.

Results of each test are shown in Table 4 to Table 6.

Table 4 shows results of an example concerning the first aspect of the present invention of Table 1.

As shown in Table 1 and Table 4, the alloy of Example 1 has a high Cu content, and tends to have a high strength, but by adopting aging treatment at 200° C. for 5 hours (retention at 200° C. for 5 hours), a hardness is adjusted to an average hardness HV of 160 or lower, and a stress remaining after retention at 180° C. for 300 hours (residual stress σr) in the test for assessing the relaxation resistance property is high.

Alloys of Examples 2 and 3 have a Cu content close to upper and lower limits in the present invention, respectively, however a hardness is in the range of a first aspect of the present invention and a residual stress σr is high.

The alloy of Comparative Example 2 has similarly a high Cu content, but by heat treatment at 180° C. for 8 hours, an average hardness HV exceeds 160 and, as a result, a residual stress σr becomes low.

Since the alloy of Comparative Example 1 has a low amounts of Cu and Mg, an average hardness HV becomes less than 130 and a residual stress σr becomes low.

Since alloys of Comparative Examples 3 to 5 have a low Cu content, and contain Ni, a residual stress σr is low.

Since the alloy of Comparative Example 6 has a low Cu content, and a high Mg content, a residual stress σr is low.

In addition, alloys of Comparative Examples 2, 5 and 6 have an average hardness HV exceeding 160, and a residual stress σr is low.

The alloy of Comparative Example 7 has a Cu content exceeding 6%, an average hardness HV exceeding 160, and a residual stress σr is low. And, a density is higher than 2.8 g/cm³.

From the forgoing results, it is seen that the cast aluminum alloy of the first aspect of the present invention in which Cu is 3 to 5 mass %, Ni is not more than 0.5 mass %, and a hardness is adjusted to HV 130 to 160 by heat treatment, exhibits the excellent relaxation resistance property.

Table 5 shows results of an example concerning the second aspect of the present invention shown in Table 2.

As shown in Table 2 and Table 5, alloys of Examples 4 to 8 are an alloy belonging to the second aspect of the present invention, contain a suitable amount of Ti, Zr and V, and a homogeneous structure in which an area ratio of a region of the aligned dendrite structure is not more than 20%. In addition, by heat treatment, a hardness is suitably adjusted. As a result, as compared with the alloy of Example 1 not containing Ti, Zr, V and the like, the further high relaxation resistance property is exhibited.

Comparative Example 9 has a higher Si amount as compared with a composition range of the second aspect of the present invention, contains Ni and P, and has a high average hardness HV. As a result, a residual stress σr is lower as compared with Examples 1 to 8.

Further, the alloy of Comparative Example 9 has a higher density of not lower than 2.8 g/cm³ as compared with alloys of Examples 1 to 8, and has a disadvantage of increasing a weight of casting.

The alloy of Example 6 is an optimal alloy in the second aspect of the present invention, and has a very isotropic network structure in which an area ratio of an aligned dendrite structure is low as not more than 5%. Further, the alloy of Example 6 has a small average longitudinal diameter of crystallized Si of not more than 3 μm, and has a structure in which an aggregation of fine eutectic Si forms a wide network skeleton. As a result, the alloy of Example 6 exhibits the further higher relaxation resistance property than that of the alloys of Examples 4, 5, 7 and 8.

Alloys of Examples 4, 5 and 7 have a greater average longitudinal diameter of crystallized Si of not less than 5 μm as compared with the optimal alloy of Example 6. The alloy of Example 8 has a slightly larger area ratio of the aligned dendrite structure of not less than 10% as compared with the optimal alloy of Example 6. For this reason, the relaxation resistance property of alloys of these Examples 4, 5, 7 and 8 is slightly inferior as compared with the optimal alloy of Example 6 as described above, but those alloys have the sufficiently higher performance as compared with previous alloys of Comparative Examples 1 to 9.

Table 6 shows results of an example concerning the second aspect of the present invention in which an essential composition is further increased than that of Table 2 as shown in Table 3, thereby, so-called recycled metal is allowed to be used.

As shown in Table 3 and Table 6, the alloy of Example 9 is an alloy manufactured by blending a recycled metal, and is adjusted so as to contain a suitable amount of Pb, Sn and Zn. Since a content is suitable, as a residual stress σr, a higher value is obtained as compared with alloys of Comparative Examples 1 to 9. Further, since the alloy of Example 9 contains Sn, Pb and Zn, it can use a recycled metal for a raw material as described above. Therefore, it has the characteristic being excellent in the recycling property. This can revolutionarily reduce an energy necessary for manufacturing an alloy, and the CO2 saving effect is extremely great.

The alloy of Comparative Example 10 is an alloy manufactured by similarly using a recycled metal containing Sn, Pb and Zn, but since a content is not suitable, a residual stress σr is remarkably reduced as compared with Example 9. From this result, it is seen that when a content of Sn, Pb and Zn is properly adjusted in the alloy of the second aspect of the present invention, an alloy having both of the recycling property and the relaxation resistance property can be realized.

(Table 4)

TABLE 4 average area ratio longitudinal of an diameter of relaxation average aligned crystallized use of resistance property hardness dendrite Si density recycled (residual stress) sample HV Adp DsL (μm) ρ (g/cm³) ingot σ r(MPa) Comparative 111 65% 7.8 2.72 yes 111 Example 1 Comparative 165 68% 2.3 2.75 no 101 Example 2 Comparative 151 4% 8 2.78 no 114 Example 3 Comparative 150 29% 6.8 2.78 no 107 Example 4 Comparative 166 29% 6.8 2.78 no 106 Example 5 Comparative 167 20% 5 2.77 no 98 Example 6 Comparative 125 65% 7.8 2.72 yes 90 Example 7 Comparative 179 30% 8.6 2.81 no 106 Example 8 Example 1 154 29% 7.5 2.75 no 135 Example 2 145 29% 7.6 2.77 no 132 Example 3 159 28% 7.9 2.79 no 134

(Table 5)

TABLE 5 average area ratio longitudinal of an diameter of relaxation average aligned crystallized use of resistance property hardness dendrite Si density recycled (residual stress) sample HV Adp DsL (μm) ρ (g/cm³) ingot σ r(MPa) Example 4 159 2% 6.5 2.75 no 144 Example 5 144 2% 6.5 2.75 no 145 Example 6 154 5% 2.4 2.75 no 151 Example 7 158 7% 6.5 2.75 no 145 Example 8 153 17% 2.4 2.75 no 143 Comparative 166 2% 8.8 2.83 no 128 Example 9

(Table 6)

TABLE 6 average area ratio longitudinal of an diameter of relaxation average aligned crystallized use of resistance property hardness dendrite Si density recycled (residual stress) sample HV Adp DsL (μm) ρ (g/cm³) ingot σ r(MPa) Example 9 156 5% 6.5 2.77 yes 137 Comparative 156 5% 6.9 2.78 yes 118 Example 10

For reference, metal microphotographs of representative alloys among the aforementioned respective cast aluminum alloys are shown in FIG. 5 to FIG. 14. 

1. A cast aluminum alloy excellent in the relaxation resistance property, comprising: 9 to 17% by mass of Si, 3 to 6% by mass of Cu, 0.2 to 1.2% by mass of Mg, 0.2 to 1.5% by mass of Fe, 0.1 to 1% by mass of Mn, and a balance consisting of Al and unavoidable impurities, wherein a content of Ni is not more than 0.5% by mass, and an average hardness is HV130 to HV160.
 2. The cast aluminum alloy excellent in the relaxation resistance property according to claim 1, wherein the average hardness is adjusted by performing, after casting, solution heating by retaining the alloy at a treatment temperature of 450 to 510° C. for 0.5 hour or longer, performing water quenching and, thereafter, performing aging treatment by retaining the alloy at a treatment temperature of 170 to 230° C. for 1 to 24 hours.
 3. The cast aluminum alloy excellent in the relaxation resistance property according to claim 1 or 2, wherein the alloy has a hypoeutectic structure in which a Si content is 9 to 12% by mass and primary crystal Si is not present.
 4. A cast aluminum alloy excellent in the relaxation resistance property, comprising: 9 to 17% by mass of Si, 3 to 6% by mass of Cu, 0.3 to 1.2% by mass of Mg, 0.2 to 1% by mass of Fe, 0.1 to 1% by mass of Mn, 0.15 to 0.3% by mass of Ti, and a balance consisting of Al and unavoidable impurities, wherein a content of Ni is not more than 0.5% by mass, and the alloy has an isotropic homogeneous structure in which a ratio of area of dendrite where 5 or more dendrite cells are aligned generally in one direction is not more than 20% in terms of an area ratio, and there is substantially no alignment of dendrites.
 5. The cast aluminum alloy excellent in the relaxation resistance property according to claim 4, further comprising 0.05 to 0.15% by mass of Zr, and 0.02 to 0.15% by mass of V.
 6. The cast aluminum alloy excellent in the relaxation resistance property according to claim 4 or 5, wherein the alloy has a hypoeutectic structure in which a Si content is 9 to 12% by mass, a P content is not more than 0.001% by mass, and primary crystal Si is not present.
 7. The cast aluminum alloy excellent in the relaxation resistance property according to claim 6, further comprising at least one member selected from the group consisting of: 0.0005 to 0.01% by mass of Ca, 0.0005 to 0.003% by mass of Na, 0.003 to 0.03% by mass of Sr, and 0.05 to 0.2% by mass of Sb.
 8. The cast aluminum alloy excellent in the relaxation resistance property according to any one of claims 4 to 7, wherein an average hardness is HV130 to HV160.
 9. The cast aluminum alloy excellent in the relaxation resistance property according to claim 8, wherein the average hardness is adjusted by performing, after casting, solution heating by retaining the alloy at a treatment temperature of 450 to 510° C. for 0.5 hour or longer, performing water quenching and, thereafter, performing aging treatment by retaining the alloy at a treatment temperature of 170 to 230° C. for 1 to 24 hours.
 10. The cast aluminum alloy excellent in the relaxation resistance property according to any one of claims 6 to 9, wherein an average longitudinal diameter of crystallized Si is not more than 5 μm.
 11. The cast aluminum alloy excellent in the relaxation resistance property according to any one of claims 1 to 10, further comprising at least one member selected from the group consisting of: 0.01 to 0.1% by mass of Sn, 0.02 to 0.15% by mass of Pb, and 0.1 to 1% by mass of Zn.
 12. The cast aluminum alloy excellent in the relaxation resistance property according to any one of claims 1 to 11, wherein a Si amount in a base Al phase is not less than 0.95% by mass.
 13. A method of heat-treating a cast aluminum alloy excellent in the relaxation resistance property, containing: 9 to 17% by mass of Si, 3 to 6% by mass of Cu. 0.2 to 1.2% by mass of Mg, 0.2 to 1.5% by mass of Fe, 0.1 to 1% by mass of Mn, and a balance consists of Al and unavoidable impurities, wherein a content of Ni is not more than 0.5% by mass, and the method comprising: performing, after casting, solution heating by retaining the alloy at a treatment temperature of 450 to 510° C. for 0.5 hour or longer, performing water quenching and, thereafter, performing aging treatment by retaining the alloy at a treatment temperature of 170 to 230° C. for 1 to 24 hours, thereby, adjusting an average hardness at HV130 to HV160.
 14. A method of heat-treating a cast aluminum alloy excellent in the relaxation resistance property, containing: 9 to 17% by mass of Si, 3 to 6% by mass of Cu. 0.3 to 1.2% by mass of Mg, 0.2 to 1% by mass of Fe, 0.1 to 1% by mass of Mn, 0.15 to 0.3% by mass of Ti, and a balance consists of Al and unavoidable impurities, wherein a content of Ni is not more than 0.5% by mass, and the method comprising: performing, after casting, solution heating by retaining the alloy at a treatment temperature of 450 to 510° C. for 0.5 hour or longer, performing water quenching and, thereafter, performing aging treatment by retaining the alloy at a treatment temperature of 170 to 230° C. for 1 to 24 hours, thereby, adjusting an average hardness at HV130 to HV160.
 15. The method of heat-treating a cast aluminum alloy excellent in the relaxation resistance property according to claim 14, wherein the cast aluminum alloy further comprises: 0.05 to 0.15% by mass of Zr, and 0.02 to 0.15% by mass of V.
 16. The method of heat-treating a cast aluminum alloy excellent in the relaxation resistance property according to claim 14 or 15, wherein the cast aluminum alloy has a hypoeutectic structure in which a Si content is 9 to 12% by mass, a P content is not more than 0.001% by mass, and primary crystal Si is not present.
 17. The method of heat-treating a cast aluminum alloy excellent in the relaxation resistance property according to claim 16, wherein the cast aluminum alloy further contains at least one member selected from the group consisting of: 0.0005 to 0.01% by mass of Ca, 0.0005 to 0.003% by mass of Na, 0.003 to 0.03% by mass of Sr, and 0.05 to 0.2% by mass of Sb.
 18. The method of heat-treating a cast aluminum alloy excellent in the relaxation resistance property according to any one of claims 13 to 16, wherein the cast aluminum alloy further contains at least one member selected from the group consisting of: 0.01 to 0.1% by mass of Sn, 0.02 to 0.15% by mass of Pb, and 0.1 to 1% by mass of Zn. 